A standard brake disk comprises a massive metallic element, typically of aluminum, having a center cup from which a flat rim projects radially. It is essential that the two faces of this disk be ground true to two parallel planes perpendicular to a center axis of the disk, which axis is perpendicular to the floor of the center cup. Such disks, and similar machine parts of similar construction, are produced in large quantity by mass-production operations but, in spite of the large-volume production, must be made to very tight tolerances.
Accordingly it is standard to grip the disk in a holder which comprises a pair of axially spaced parts which can be rotated and at least one of which can be axially displaced between a holding position pressing the disk against the other element and a release position clear of the other element and allowing disks to be changed. At least one of these parts is driven and a pair of grinding stones adjacent the holder can be brought into axial engagement with opposite faces of the disk in the holder so as to fine-grind its faces as it and the holder rotate at high speed.
The actuator for the movable holder element or for both elements is maintained under pressure to clamp the disk tightly axially between the holder elements. Thus there is in effect a force loop having one side at the two holder elements and passing through the frame of the machine. In use the machine vibrates and is subject to considerable thermal expansion and contraction, so that the clamping force varies. This can lead to slipping of the disk and spoiling of it as it becomes canted between the stones. Furthermore the clamping force is applied to the bearings carrying the holder parts in the frame of the machine so that these bearings are subjected to considerable stress, shortening their service lives even when they are of very heavy-duty construction.